VOL. 67, No. S
MAY 1967
Psychological Bulletin
SENSORY ADAPTATION AND BEHAVIORAL COMPENSATION
WITH SPATIALLY TRANSFORMED VISION AND HEARING
R. H. DAY
G. SINGER
Monash University
University of Sydney
An analysis of spatial transformations of perceived space is made in terms of
angular and parallel modifications of the median, horizontal, and frontal
planes of O, and the perceptual and behavioral outcomes of such transformations examined. It is argued that there are 2 independent outcomes: behavioral
compensation and sensory spatial adaptation with aftereffect. The 1st can be
regarded as a special case of motor learning similar to that studied in early
investigations with frontal plane transformation (mirror tracing), and the
2nd is essentially similar to spatial adaptation which may occur with appropriate nontransformed stimulation. Both effects can occur simultaneously in
the same direction, but the experimental data presented show that they can be
studied independently. The effects observed by Ivo Kohler are treated as
special instances of sensory adaptation which occur with transformations
dependent upon sense-organ position and movement. The felt-position hypothesis and the reafference theory proposed by Held are shown to be reinterpretable in terms of motor learning and transfer of learning. Various
methodological issues in the investigation of motor learning and sensory
adaptation are examined.
If before entering the eye light is spatially
transformed by means of an optical system
(prism, lens, mirror), visually guided behavior (reading, walking, card sorting) which is
initially disrupted exhibits a progressive return to pretransformation normality. In addition, under certain spatial transformation conditions, visual appearances (shape, size, direction) are also reported to undergo change
and to exhibit a tendency to return to pretransformation appearances. Analogous changes
also occur when hearing is spatially transformed by means of "pseudophonic" systems.
Since Czermak (1855), Helmholtz (1866),
and Wundt (1898) drew attention to these
effects a variety of explanations have been
proposed in terms of perception (Kohler,
1964; J. G. Taylor, 1962), motor learning
(Smith Si Smith, 1962), reafference (Held &
Freedman, 1963), and "felt position" (Harris,
1965). In addition to clarifying the nature of
spatial transformations, the purpose of this
paper is to show that there are two independent outcomes, sensory spatial adaptations and behavioral compensation, which
under some conditions may occur together in
the same direction. The first is a purely sensory effect identical to that which occurs
without transformation (Gibson, 1933, 1937;
Kohler & Wallach, 1944), and the second is
a special case of motor learning identical to
that investigated in the early mirror-tracing
studies (Starch, 1910). The recent interpretation of the second effect in terms of felt
position merely deals with learning from a
phenomenological viewpoint. It is argued
that failure to recognize the simultaneous occurrence of these independent outcomes of
transformed vision and hearing has resulted
in inappropriate procedures and a tendency
to treat both as specific characteristics of
transformed conditions rather than as instances of effects which occur under a variety
of conditions without transformation.
NATURE OF SPATIAL TRANSFORMATIONS
Despite considerable recent research there
has so far been no attempt to describe or to
classify the classes, directions, extents, and
complexities of transformations. This omis-
307
308
R. H. DAY AND G. SINGER
sion not only makes comparisons of studies
difficult, but also obscures relationships between early experiments concerned with motor learning and more recent investigations
whose data have been differently interpreted.
Perceived space can be usefully described
in terms of three hypothetical planes: the
median (sagittal), horizontal, and frontal (or
coronal) planes, which for present purposes
can be conceived as intersecting at right
angles at the center of the head. When the
observer is vertical, the median (straight
ahead) and frontal (right-left) planes are
vertical and the horizontal plane is parallel
with the ground plane. Spatial transformations which may vary in class, direction, extent, and complexity can be described in
terms of these three planes and those parallel
to them, the median, horizontal, and frontoparallel planes.
Classes of Transformation
There are two classes of spatial transformation, which will be termed angular and
parallel. Angular transformation occurs when
the optical or pseudophonic system rotates the
plane or planes about their transverse or
longitudinal axes. A wedge prism with base
vertical rotates the median plane right or
left about its transverse (i.e., vertical) axis
so that objects normally straight ahead now
appear laterally displaced. A double Doveprism system rotates both the median and
horizontal planes together about their longitudinal (i.e., horizontal) axes so that normally
vertical and horizontal contours appear slanted
(see Figure 1). A pseudophone with microphones lying anterior to one ear and posterior
to the other serves to rotate the median plane
about its transverse axis, and microphone
placement above one ear and below the other
results in transformation of the same plane
about its longitudinal axis. A mirror before
the eyes rotates the frontal plane through
180° and the degree of rotation can be varied
by slanting the mirror.
Parallel transformations occur when one or
more of the three planes are displaced in a
linear fashion without rotation about either
axis.1 Thus Weinstein, Sersen, and Weinstein
1
Magnifying and minifying lenses serve to increase
and decrease respectively the magnitude of retinal
(1964) used a mirror system arranged so
that objects at a certain distance appeared to
be farther away. Although parallel transformations of auditory space have not been reported so far, they are entirely feasible. Two
microphones the same distance apart as the
ears but mounted above the head so that both
are to the same side of each ear would presumably achieve a parallel transformation of
the median plane. Placing both an equal distance above or behind each ear would produce a parallel transformation of the auditory
horizontal and frontal planes, respectively.
Direction and Extent of Transformation
Angular transformation can vary between
0° and 360° in either direction about both
axes, and parallel transformations can vary
to the limits of visual or auditory acuity. The
mirror systems used by Kohler (1964) transformed either the median or horizontal plane
through 180° so that objects normally right
appeared left and those originally above appeared below. Ewert (1930), Snyder and
Pronko (1952), and Stratton (1896, 1897)
introduced double-convex lens systems so that
both median and horizontal planes were simultaneously transformed through 180° about
their longitudinal axes. A mirror directly before the eyes transforms the frontal plane
through 180° longitudinally so that objects in
back appear in front (but objects in front are
obscured by the mirror). Stratton (1899)
used two mirrors to transform angularly the
frontal plane through 90°. In most recent
studies, the transformations have been relatively small (10-20°) and most usually of
the median plane. The systems used by
Young (1928) and Willey, Inglis, and Pearce
(1937) transformed the auditory median
plane through 180° about its transverse axis,
and, more recently, smaller angular transformations of the same plane have been induced
(Freedman & Zacks, 1964; Held, 19SS).
Complexity of Transformations
The simplest transformation is small in
extent involving a single plane. Complexity
projections thus changing apparent size and distance. In this manner (as when viewing through the
eyepiece or mouth of a telescope) parallel transformations of fronto-parallel planes are achieved.
SENSORY ADAPTATION AND BEHAVIORAL
of transformation can vary as a function of
class combinations (angular and parallel),
number of planes simultaneously transformed,
and extents. Probably the most complex
transformation investigated so far is that used
by Snyder and Pronko (1952) involving 180°
angular changes in the median, horizontal, and
frontal planes. This was achieved by means
of a double-convex lens system and a mirror.
Area, oj Spatially Transformed Field
In vision, the angle subtended by the transformed field may vary from the total visible
field to a small part of it. Prisms worn close
to the eyes in spectacle frames transform the
total field, whereas a prism set in a screen
and viewed from a distance alters only that
part of the field bounded by the prism. On
viewing into a large mirror the total field is
transformed with respect to the frontal plane,
but a small mirror held in the hand transforms
only that area bounded by the mirror.
In hearing, as in vision, spatial transformations may vary in class, direction, extent, and
complexity by suitable positioning of microphones or other devices. Because of the temporal basis of auditory localization, however,
the total field is always transformed.
REPRESENTATIVE STUDIES
Helmholtz (1866), using prisms worn before the eyes, angularly transformed the median plane and observed progressive reduction
in reaching errors and the occurrence of errors in the opposite direction when the prisms
were removed. Wundt (1898), with similar
transformation, commented on changes in the
apparent direction of objects during and after
the transformation period. Wundt observed
that whereas objects initially appeared angularly displaced, this effect progressively diminished. In probably the best known of all
spatial transformation studies, Stratton (1896,
1897), rotated the median and horizontal
planes together through 180° about their
longitudinal axes and noted progressive improvement in initially disrupted locomotor and
manipulative behavior. The purely perceptual
features of this long-term transformation have
remained controversial although later experiments (Snyder & Pronko, 1952) have sug-
COMPENSATION
309
gested that there were no marked perceptual
changes. Later, Gibson (1933) used wedge
prisms to give a double median plane transformation about the transverse axis so that
straight edges were curved, and he observed
that this perceived curvature progressively diminished. On removal of the prisms, straight
edges appeared curved in the opposite direction. In later experiments, Gibson dispensed
with prisms and established that an inspected
field of curved lines results in the same effects
as those of prismatic curvature. Subsequent
experiments (Gibson & Radner, 1937) showed
that essentially similar effects occur with
slanted lines, thus confirming an earlier finding by Verhoeff (1902).
Independent of but parallel with these
visual transformation investigations were a
series of studies of motor learning which used
180° transformations of the frontal plane to
produce a novel learning task. Beginning with
Starch (1910), the typical "mirror-drawing"
or "mirror-tracing" apparatus prevented a
direct view of the hand which was viewed in
a mirror so that only part of a fronto-parallel
plane was rotated through 180°. The observer's task was that of drawing, writing, or
tracing a pattern while viewing the hand in
the mirror. The task was further developed
and used by Siipola (1935) with the mirror
arranged so that the median and horizontal
planes were also transformed. As in the other
studies reviewed, over a number of trials there
was a gradual improvement in accuracy.
It can also be noted that while in most
experiments it has been found that errors in
reaching, pointing, and marking occur in the
opposite direction after the transformation
period (Held & Hein, 1958; Held & Schlank,
1959), they have also been observed to occur
in the same direction as the transformation
(Weinstein, Sersen, & Weinstein, 1964). This
effect is discussed below.
In summary, investigations of the effects of
spatial transformation have variously emphasized the behavioral or perceptual outcomes.
While Gibson (1933, 1937) was wholly concerned with visual perceptual phenomena,
Starch (1910) and Siipola (1935) were solely
interested in changes in motor behavior.
Stratton (1896, 1897) reported both perceptual and motor changes as did Kohler (1964).
310
R. H. DAY AND G. SINGER
Visual and Auditory Adaptation
The perceptual outcomes of spatially transformed vision can for the most part be reproduced using appropriate stimulus conditions without transformation. After Gibson
(1933) demonstrated apparent diminution of
prism-induced curvature with subsequent opposite curvature when the prism was removed,
he showed that inspection of curved and then
straight lines resulted in the same effects. The
prisms induce a double transformation about
the longitudinal axis of the median plane so
that the retinal projections of straight lines
lying in a fronto-parallel plane are curved. A
normally viewed curved line of course results
in a similar retinal projection. Wundt (1898),
however, observed that with the median plane
transformed about its transverse axis by
means of prisms there was a progressive diminution of the apparent deviation of objects
from the straight-ahead. If a field of lines
lying in the horizontal plane and parallel to
the median plane is viewed through a wedge
prism, they appear slanted to one side. With
prolonged viewing through the prism the lines
appear progressively more straight-ahead. This
observation made by one of the authors is
essentially that made by Wundt (1898). It
has also been observed that a field of lines
slanted away from the median plane and
lying in the horizontal plane appear progressively more straight-ahead with prolonged
normal viewing.
Essentially similar effects to those demonstrated by Gibson (1933) and Wundt (1898)
occur with angular transformations of the
frontal plane. Wertheimer (1912) used a
large slanted mirror so that when an observer
stood before it the floors and walls of a room
appeared slanted. This apparent slant diminished with prolonged viewing in the mirror.
Although Gibson (1952) has disputed this
finding, he (Gibson & Bergman, 1959) observed an essentially similar effect with a normally viewed tilted surface. Following inspection of a textured surface a subsequently presented vertical surface appeared slanted in the
opposite direction (Gibson & Bergman, 1959).
Kohler and Wallach (1944) showed that
following fixation of a figure, the apparent
size of a slightly larger figure increased com-
pared with its apparent size prior to fixation.
Similarly, the apparent size of a slightly
smaller figure was decreased. It would be expected, therefore, that if the retinal projection
of an object were reduced in size by means of
a minifying optical system, subsequently
viewed objects would appear larger. Rock
(1965), using a convex mirror to minify retinal projections, has demonstrated that such
changes in judged size occur.
In general terms, visual perceptual changes
occur with spatially transformed space. If
the same stimulus conditions are reproduced
without transformation (e.g., actually curved
lines as opposed to prismatically curved retinal projections), essentially similar changes
occur.
Although spatial adaptation and aftereffect
have been observed to occur in hearing (Bartlett & Mark, 1922; Flugel, 1921; Krauskopf,
1954; M. M. Taylor, 1962) with a sound
source to one side of the median plane, the
same effects have not been reported with spatially transformed hearing. There is no reason
to suppose, however, that similar effects would
not occur if the same conditions were produced by a pseudophonic device. The experiments with spatially transformed hearing reported by Held (1955) and Freedman and
Zacks (1964) were concerned with behavioral
changes.
Behavioral Compensation
If a wedge prism transforms the median
plane 20° right about its transverse axis, the
observer reaching for an object quickly will
at first move too far to the right. After several attempts, however, accuracy is achieved;
and when, without visual guidance, the observer points straight-ahead, his direction of
pointing will be to the left of his pretransformation straight-ahead. The leftward compensation for rightward angular transformation
persists. Similar effects can be easily demonstrated with a variety of directions, extents,
and complexities of transformation. The essential feature of these effects is a change in
the direction of responses such as pointing,
reaching, marking, walking, or card sorting
during the transformation period such that
compensation is made for the effects of transformation. The posttransformation responses
SENSORY ADAPTATION AND BEHAVIORAL
in the absence of visual or auditory guidance
tend to be in the same direction as those developed during the transformation period.
Independent Occurrence oj the Two Effects
Gibson and Radner (1937) showed that
adaptation to slanted lines achieved a maximum when the field of lines was slanted at
15-20° and was negligible at 45°, a finding
which has been confirmed by Logan (1962)
and by Morant and Seller (1965) for prismatically slanted lines. In a recent unpublished experiment by the present authors,
a physically horizontal line was viewed
through a double Dove-prism system (Figure
COMPENSATION
311
1) so that the median and horizontal planes
were angularly transformed about their horizontal axes. In one condition, the prisms were
arranged so that the retinal projection of the
line was slanted 20°; and in the other condition, at 45°. The mean aftereffect (difference
between pre- and posttransformation adjustments to horizontality with normal viewing)
for two groups of 10 observers each was
1.57° (p<.0l) for the 20° slant and .16°
(p > .05) for 45°. The transformation period
was 3 minutes. Thus the visual adaptation
effect is negligible at 45° transformation and
significant for the smaller slant.
FIG. 1. Apparatus used to induce spatial adaptation by visual inspection of an optically slanted (but objectively horizontal) bar and behavioral compensation by observation of the hand moving across the
optically transformed bar.
312
R. H. DAY AND G. SINGER
If, on the other hand, the observer moves
his extended hand across a physically horizontal bar while viewing both hand and bar
(Figure 1) and sets it so it seems horizontal
at IS-second intervals during the 3-minute
period, he will adjust it to compensate for
the visual transformation in the opposite direction to the slant. This compensation for
visual transformation persists so that it occurs
when the observer makes an adjustment to
apparent horizontality without visual guidance
after the transformation period. In an extension of the experiment, two groups of 12 observers adjusted the bar so that it seemed
horizontal at IS-second intervals during the
3-minute transformation period, observing the
bar and their hand moving across it. The
prisms were arranged so that the retinal projection of the horizontal bar was slanted 20°
for one group and 45° for the other. The
aftereffect (difference between pre- and posttransformation adjustments without visual
guidance) was 6.86° for the 20° transformation and 8.23° for the 45° transformation.
This latter finding is similar in principle to
that reported by Harris (1965) in which the
observer moved his viewed hand across a
prismatically curved (but physically straight)
line.2 It is clear from the data reported here,
however, that the visual and behavioral effects
are independent. Whereas visual adaptation
with aftereffect is negligible at 45° of prismatically induced slant, the behavioral effect
is substantial. At 20° slant, the visual effect
is larger than at 45°, while the behavioral
effect is larger at 45° than at 20°. Since
there is no visual effect at 45°, it seems unlikely that this phenomenon is due to changes
in the registration of eye movements as suggested by Harris (1965) for prismatically induced curvature. Such a hypothesis would
predict that the visual effect would be greater
at 45° than at 20°. There is, of course, no
essential difference between visual adaptation
deriving from the inspection of prismatically
curved and prismatically slanted lines.
2
Learning to move the hand across an objectively straight but prismatically curved line is, of
course, essentially similar to a mirror-tracing task.
Whereas mirror tracing usually involves a 180°
transformation of the frontal plane, Harris's task
involved a double median plane transformation
about its longitudinal axis.
Simultaneous Occurrence of the Two
Effects
Under a variety of spatial transformation
conditions, sensory adaptation and behavioral compensations can occur simultaneously
in the same direction. Consider a wedge prism
which transforms the median plane through
20° to the right set into a sheet of plate glass
with the prism surface flush with the glass
and the whole forming the top of an opensided box. If a rod is placed a few inches below the underside of the glass, that section
directly beneath the prism is displaced to the
right so that the rod appears to be discontinuous. A cover with a line the same width
as the rod and coincident with it can be placed
over the glass and prism. With fixed head position, the observer fixates the rod at the center of the prism for 2 minutes, after which the
cover is quickly replaced and the line fixated
at the point coincident with the previous fixation point. Since the central section of the
rod was prismatically displaced right, the same
section of the line appears displaced left. This
is a typical visual spatial aftereffect. Now, if
while viewing the rod the observer quickly
reaches beneath the glass and touches the
section beneath the prism 10 times during a
2-minute period, he will at first reach too far
right. After several responses, accuracy will
be achieved; and when the cover is replaced
and the observer is required to touch its
underside directly beneath a point coincident
with the center of the prism, his response will
be too far left. That is,' compensation for
transformed vision during the transformation
period persists and is revealed in posttransformation responses without visual guidance. The
point to note is that the visual spatial aftereffect and behavioral aftereffect both deriving from median plane transformation are in
the same (leftward) direction.
The independence of these two effects can
be demonstrated by requiring the observer
during the second part of the experiment to
reach out quickly and touch the rod, allowing
about 3 seconds for the response and closing
his eyes for about 20 seconds between each
response. In this case, behavioral compensation will be marked but the visual effect negligible since there is insufficient time for it to
develop fully and what does develop will dis-
SENSORY ADAPTATION AND BEHAVIORAL
sipate during the periods when the eyes are
closed (Hammer, 1949; Oyama, 19S3). With
spatial transformation of the total visual field
by means of prisms worn before the eyes for
a long period, both visual and behavioral
adaptation with subsequent aftereffects on
removal would be expected to occur.
Whether or not visual adaptation occurs in
addition to behavioral compensation depends
on the type and extent of spatial transformation. For example, for the reasons already
outlined, visual adaptation with aftereffect
would not be expected with the median plane
angularly transformed 45° about its longitudinal axis. Since with 180° transformations
of the median or horizontal planes (or with
both, as studied by Stratton, 1896, 1897),
the straight-ahead remains straight ahead and
vertical and horizontal edges remain so;
adaptation with aftereffect would not be expected.
Behavioral Compensation and Kinestketic
Aftereffect
In addition to vision and hearing, sensory
spatial adaptation with consequent aftereffect
occurs following prolonged kinesthetic stimulation (Day & Singer, 1964; Gibson, 1933;
Kb'hler & Dinnerstein, 1947). Under certain
conditions a kinesthetic aftereffect and behavioral compensation may occur together in
the same direction. For this reason it is necessary to distinguish between them.
If a blindfolded observer moves his extended hand from side to side across a slanted
edge, a subsequently presented horizontal edge
is judged to be slanted in the opposite direction. If, however, the observer is required to
adjust the edge so that it feels horizontal, he
will set it in the same direction as the original
slant. Movement across an objectively horizontal edge does not result in an aftereffect
if the poststimulation task is that of adjusting the edge to apparent horizontality (Day
& Singer, 1964). With the apparatus shown
in Figure 1, the actual slant of the bar and
optical slant (i.e., median and horizontal plane
transformation) can be varied independently.
In a recent series of experiments8 in which
bar slant and optical slant were independently
8
These experiments were conducted by Mrs.
Margaret Austin, University of Sydney, 1966.
COMPENSATION
313
varied in both directions, the observer moved
his hand from side to side across the bar while
viewing it, after which a nonvisual posttransformation adjustment to apparent horizontality was made. The results from these experiments show that the kinesthetic aftereffect
and behavioral compensation may summate
or cancel as a function of the extent and direction of bar and optical slants. If, for example, the bar was slanted left and optical
slant was right, the two effects summed. If
both bar and optical slant were in the same
direction, the net effect was zero since the
kinesthetic aftereffect and behavioral compensation occur in opposite directions, as Harris
(196S) has implied. If the bar was objectively
horizontal but optically slanted, a pure measure of behavioral compensation was obtained
since, as pointed out above, the kinesthetic
aftereffect does not occur under this condition.
It seems likely that in some studies the
occurrence of a kinesthetic aftereffect has affected the results which have been interpreted
in terms of "negative behavioral compensation." Weinstein, Sersen, and Weinstein
(1964) attempted to repeat an experiment by
Held and Schlank (19S9) and obtained posttransformation responses opposite in direction
to those originally obtained. A mirror system
was arranged to give a parallel transformation of the frontal plane so that when the
resting hand was viewed it appeared at a
greater distance from the body than when
normally viewed. During the transformation
period, the hand was moved rapidly (68
movements per minute) toward and away
from the body. Before and after this 6-minute period the observer was required to mark
a point without viewing his hand. The range
of movement of the hand and the position of
marking strongly suggest that a kinesthetic
aftereffect occurred in a direction opposite to
behavioral compensation.
NATURE OF SENSORY ADAPTATION AND
BEHAVIORAL COMPENSATION
Sensory Adaptation
The essential effects of optical and pseudophonic transformation are changes in the spatial properties of stimulation similar to those
which can be achieved by appropriate pat-
314
R. H. DAY AND G. SINGER
terns without transformation. The processes
underlying the effects of protracted stimulation produced by either means, however, are
far from clear. Although a number of theories
have been proposed to explain spatial aftereffects (Gibson, 1937; Kohler & Wallach,
1944; Osgood & Heyer, 1952), none is entirely satisfactory. It is sufficient to note for
present purposes, however, that spatial aftereffects in vision occur with retinal stabilization of the projected image (Ganz, 1964),
thus ruling out ocular movement as a determinant. Howard and Templeton (1964)
demonstrated that effects from slanted lines
cannot be attributed to ocular torsion as
proposed by Ogle (19SO).
Interocular transfer. It is well established
(Gibson, 1933; Kohler & Wallach, 1944)
that following appropriate stimulation of
one eye with the other occluded, a spatial
aftereffect occurs when the occluded eye is
used for testing. The occurrence of the
effect in the previously nonstimulated eye is
generally referred to as interocular transfer.
It would be expected that if spatial adaptation with aftereffect consequent upon transformed vision represents the same effects as
with appropriate nontransformed vision, interocular transfer would also occur with transformed stimulation. That this is so has been
demonstrated by Ebenholtz (1966), Hajos
and Ritter (196S), and Pick, Hay, and Willoughby (1966). The question of whether or
not such demonstrations of interocular transfer provide evidence for the central origin of
the processes involved remains unsettled
(Day, 1958).
Behavioral Compensation
It is argued here that changes in behavior
concomitant with transformed visual and
auditory input are a special case of motor
learning essentially similar to and exhibiting
the same characteristics as the learning process observable with the pursuit-rotor, fingermaze or mirror-tracing apparatus. To this extent it seems unnecessary to attribute special
properties to behavior deriving from transformed sensory input as has been common in
recent contributions (Harris, 1965; Held &
Freedman, 1963). That is, it seems unneces-
sary and unparsimonious to regard changes
in behavior deriving from transformation as
essentially different from changes occurring
in a variety of alternative motor-learning
situations.
If an observer wearing wedge prisms which
transform the median plane transversely 20°
right regards an object which is objectively
straight-ahead, it appears 20° to the right. In
reaching quickly for the object, the observer
may at first miss it but after several attempts
he will reach accurately. If reaching is continued, proprioceptive information for a particular direction of limb movement (previously associated with the nontransformed median plane) and visual information become
associated. In brief, an appropriate response
pattern to the spatial properties of stimulation is learned. When the prisms are removed
and the observer reaches straight ahead without visual guidance, the response deviates left
of the actual straight-ahead point. Since the
original proprioceptive information for
straight-ahead has become associated with 20°
right, the new nontransformed straight-ahead
(without visual guidance of the limb) must
of necessity be to the left. In this sense, behavioral compensation is representative of the
learning process which occurs in a wide variety of situations and is subjected to extensive
enquiry. An appropriate response pattern develops in relation to a certain pattern of stimulation. Presented with a target moving rapidly in a circular path, as with the pursuitrotor, the individual, after initial inaccuracies,
exhibits an appropriate response pattern which
improves with practice. If the frontal plane is
transformed through 180° by means of a
mirror placed before the observer and he is
required to trace a star pattern while observing his hand in the mirror, a typical learning
curve can be plotted on the basis of either
time taken or number of errors per trial. If
the median plane is less drastically transformed through 20° and the observer is required to reach for an object while observing
his hand, a typical learning curve results
(Hamilton, 1964). The main difference between early studies of this process (Siipola,
1935; Starch, 1910) and more recent investigations (Held & Freedman, 1963) is that
the early experiments did not make use of
SENSORY ADAPTATION
AND BEHAVIORAL
pre- and posttransformation tests to measure
the transfer of learning to nontransformed
conditions, and the term "sensorimotor adaptation" has been coined in referring to the
change. Further, in recent investigations
there has been a failure to distinguish clearly
between the purely sensory (adaptive) phenomena and learning or to recognize that the
frequently used median plane transformations
achieved by wedge prisms are identical in
principle to the frontal plane transformations
deriving from mirrors. The classical mirrortracing procedure is a standard procedure for
demonstrating the course of motor learning
and bilateral transfer of learning. Although
the learning is more rapid (since the transformation is smaller and less complex), median plane transformations or any other spatial transformation would serve as well. Although the evidence strongly suggests that
changes in behavior with transformation represent a motor-learning process, the term "behavioral compensation" can be conveniently
used in referring to learning under these conditions.
Rate oj behavioral compensation. The evidence suggests that the rate of motor learning with transformed input varies as a function of both the extent and complexity of
transformation and the nature of the responses. In those studies involving 180°
transformations of median and horizontal
planes about longitudinal axes (Ewert, 1930;
Snyder & Pronko, 1952; Stratton, 1896, 1897)
and involving complex response patterns
such as perambulation and card sorting, the
time to achieve pretransformation efficiency
was protracted, taking up to 2 or 3 days.
Learning with simpler and less extensive
transformations involving less complex responses such as reaching or marking is relatively rapid, occupying a few seconds (Hamilton, 1964). So far, there has been no systematic study of the effects of extent, direction,
and complexity of transformation on motor
learning, although Siipola (193S) investigated
the effects of 180° transformation of the
frontal and horizontal planes singly and together. She found no difference between the
three conditions. The accumulated data from
a number of studies indicate, however, that
the extent of transformation may be a criti-
COMPENSATION
315
cal variable affecting rate of learning to a set
criterion.
Intermanual transfer oj behavioral compensation. Kalil and Freedman (1966) found
that after viewing one hand with transformed
vision behavioral compensation occurred in
the other hand in a nonvisual posttransformation test. This finding was not confirmed by
Harris (1963) or Mikaelian (1963). Hamilton (1964) noted intermanual transfer with
unrestricted head and body movement but
found that it did not occur with restricted
movement. Intermanual or, as it was called
earlier, bilateral transfer, is well established
with 180° transformation of the frontal plane
in the typical mirror-tracing experiment
(Ewert, 1926; Siipola, 1935). In fact, it has
also been found to occur between hand and
foot (Bray, 1928). Since the recent investigations of intermanual or bilateral transfer have
been restricted to relatively small spatial
transformations usually of the median plane
whereas the earlier studies employed 180°
changes, it seems that transformation extent
may be a determinant of such transfer. A
further point of difference between early and
recent studies concerns the procedure for
measuring transfer. The older procedure
(Ewert, 1926; Siipola, 1935) involved a series
of trials viewing one hand with transformed
vision followed by trials with the other hand
under the same conditions. More recent studies (Harris, 1963; Mikaelian, 1963) have investigated transfer from transformed vision
of one hand to a nonvisual test with the other.
A double transfer is involved in this procedure—transfer between a transformed visual
condition and a nonvisual condition and
transfer between one hand and the other. It
is probable that the earlier transfer studies
constituted a more sensitive test of transfer of
motor learning between limbs and were
sounder in terms of experimental design. The
"double transfer" procedure must lead to a
confounding of transfer from visual to nonvisual conditions with transfer from one limb
to the other.
Intermodal transfer. Harris (1963) demonstrated that following behavioral compensation for median plane transformation, the
compensation occurred in pointing to the direction of a sound source. Since visual and
316
R. H. DAY AND G. SINGER
auditory directions are normally in accord
(e.g., the telephone is both seen and heard
to the left or right), it follows that if new
motor responses are learned with transformed
vision they would also be manifest during a
nonvisual posttransformation test with an
auditory stimulus. So far, the opposite transfer from hearing to vision has not been demonstrated although it would clearly be of interest to do so.
Further Evidence for Behavioral Compensation as a Special Case of Motor Learning
In a recent experiment (Day & Singer,
1966) using the apparatus shown in Figure
1, the number of trials during the transformation period and during the posttransformation phase were systematically varied. The bar
which was objectively horizontal was optically transformed through 20° and the observer's task was that of adjusting it to the
apparent horizontal. Nonvisual posttransformation adjustments were compared with those
made similarly before transformation. Three
groups made a single adjustment to the horizontal while viewing the angularly transformed bar, and three further groups made
seven adjustments. While two groups (1 and
7 trials) made IS posttransformation adjustments at 1-minute intervals, two further
groups (1 and 7 trials) made a single adjustment after 7 minutes, and the remaining
""Gp.4
two groups (1 and 7 trials) made a single
adjustment after IS minutes. The data from
this experiment are plotted for the six conditions in Figure 2. It is clear from the graph
that the frequency of transformation trials
determines the degree of learning and that
dissipation of the learned response during the
posttransformation period is accelerated as a
function of the number of posttransformation
trials without visual guidance. If the transformation period is regarded as a training period during which new relationships between
stimulus and response are learned, then the
posttransformation phase can be treated as an
extinction period. During this latter period it
would be expected that in the absence of transformed stimulation the recently acquired responses would give way to previously learned
and more firmly established responses.
METHODOLOGICAL ISSUES
By far the most common procedure in investigating sensory spatial adaptation and behavioral compensation is that involving tests
before and after a period of spatial transformation. The difference between pre- and posttransformation measures serves as an index of
either visual or auditory adaptation or of behavioral compensation. The use of this procedure raises two questions—that concerning
the appropriateness of the tests to reveal one
or the other effects, and that concerning
transfer of learning from the transformation
period to the posttransformation test.
Appropriateness of Pre- and Posttransformation Tests
£3-
0 1 2
3 4 6 6 7 8 9 10 11 12 13 14
INTERVAL AFTER EXPOSURE(Mia)
FIG. 2. Dissipation of behavioral compensation
under two conditions of transformation (1 and 7
trials) and three conditions of posttransformation
(15 trials at 1-minute intervals, 1 trial after 7
minutes and 1 trial after 14 minutes).
It has been pointed out that both sensory
spatial adaptation and behavioral compensation may occur simultaneously. Whether one
or both effects are revealed depends on the
nature of the pre- and posttransformation
tests. If after a period of wearing wedge
prisms to transform the median plane through
its transverse axis the observer is required
to point straight ahead without viewing his
hand and arm, then behavioral compensation
will be exhibited. If, on the other hand, the
observer is required to report when a field of
lines in the horizontal plane appears straight
ahead, then a perceptual change will be revealed.
SENSORY ADAPTATION AND BEHAVIORAL
In a recent experiment, Mikaelian and Held
(1964) used a dual prism system to transform spatially the median plane through its
longitudinal and transverse axes and the horizontal plane through its transverse axis. The
outcome of this complex transformation was
that a point normally straight ahead was displaced left and upward and a normally vertical line was rotated into a slanted position.
There were two tests before and after transformation: adjustment of the line to apparent
verticality and rotation of the body by leg
movement until a point of light appeared
straight ahead. The first is a perceptual
test and the second is a behavioral test.
Using the conditions of Mikaelian and Held
(1964), it would be possible to devise
a behavioral test in relation to the slanted
line (e.g., pointing to the top of the line) and
a perceptual test for the straight-ahead (e.g.,
adjusting a point to appear straight ahead).
As a matter of fact, McLaughlin and Rifkin
(1965) have shown that following transverse
angular transformation of the median plane, a
spatial aftereffect occurs in adjusting a
pointer to the straight-ahead. It is clear that
appropriate behavioral and perceptual tests
could be devised for the complex transformation used by Mikaelian and Held (1964)
such that behavioral compensation and sensory spatial adaptation could be shown to
occur for the two aspects of transformation
separately investigated.
In a large number of recent investigations,
perceptual tests have preceded and followed
a transformation period during which the
observer was required to engage in various
activities. Thus, Ebenholtz (1966), using
angular transformation of median and horizontal planes, required observers to walk
through long corridors and to undertake various manipulative tasks (jigsaw puzzles, dart
throwing). The pre- and posttransformation
tests involved judgments of verticality. It is
highly likely that if the observers had merely
inspected a field of vertical lines for the same
period of time the same results would have
been obtained since spatial adaptation is dependent on stimulation rather than motor activity during the transformation period. The
same argument applies in the case of an experiment by Freedman and Stampfer (1964)
COMPENSATION
317
in which judgments of the auditory straightahead were made before and after a period of
pseudophonically transformed hearing during
which the observer walked about. Had the observer been seated for the same period the
same results probably would have occurred.
Transfer from Transformation to Test Phase
The experimental situation in which an observer learns to make certain responses with
spatially transformed vision or hearing followed by responses with normal sensory input (but usually with his responding limb
occluded) is similar in many respects to
many transfer-of-learning experiments. The
observer learns to make appropriate responses
with one form of sensory input and later responds with another. The posttransformation
responses (reaching, pointing, marking) tend
to be in the direction of those developed during the transformation phase. That is, responses learned under one condition transfer
to another. In this sense, it would be expected
that the magnitude of behavioral compensation as revealed by the difference between preand posttransformation tests would be a function of those variables known to affect transfer of learning. The relationship between
transformation and posttransformation phases
in terms of task similarity, relative task difficulty, degree of learning in the transformation phase, and time interval (Ellis, 1965)
would be expected to affect compensation.
The data from Day and Singer (1966) which
indicated that one trial during the transformation phase resulted in less compensation
than seven trials are in agreement with a
transfer-of-learning interpretation. Further
evidence in support of this view was provided
by Freedman, Hall, and Rekosh (1965) who
showed that similar activities during transformation and posttransformation phases resulted in greater transfer than dissimilar activities.
THEORETICAL ISSUES
In view of the distinction which has been
made between sensory adaptation to and behavioral compensation for spatially transformed vision and the identification of the
latter as motor learning, the role of active and
passive responding (Held & Freedman, 1963)
318
R. H. DAY AND G. SINGER
and the felt-position hypothesis
1965) can be considered.
(Harris,
Active and Passive Responding during
Transformation
Held and his associates (Held & Freedman,
1963; Mikaelian & Held, 1964) proposed that
the self-induced movement is a fundamental
determinant of what they called sensorimotor
adaptation to spatially transformed input.
Recent experiments have shown, however, that
for some conditions of transformation, externally induced or "passive" movement during
transformation is as effective as self-induced
or "active" movement (Pick & Hay, 196S;
Singer & Day, 1966b; Templeton, Howard,
& Lowman, 1966). In a recent study, (Singer
& Day, 1966a) it was shown that when the
passive, resting limb was viewed through a
wedge prism, behavioral compensation occurred if the observer was required to judge
the position of his limb in terms of a moving
scale located between prism and eye. If, on
the other hand, the observer merely viewed
his limb without judging position, compensation was negligible.
If, as argued here, changes in behavior with
spatial transformation represent learning, it
follows that the observer, in order to compensate for the altered spatial input, must be
given an opportunity to learn. Although some
learning would be expected to occur as a
result of merely observing the resting limb or
through observing another individual learning
(Siipola, 1935), maximum learning would be
more likely to occur when the observer actually responds. It does not seem at all surprising that when the observer's responses are
restricted, as when he is moved about in a
wheelchair (Held & Bossom, 1961), or when
the movements are induced by an external
force, he learns less. It has been pointed out
that the occurrence of responses learned during the transformation phase in the posttransformation phase can reasonably be regarded as an instance of transfer of learning.
If little learning takes place during transformation as a consequence of "passive" movement (i.e., limited opportunity to learn) and
the responses in the two phases are markedly
different, it is to be expected that little or no
transfer would be manifested.
There is a further point which arises in
connection with Held's theory. Proprioceptive
feedback, which is central to his argument, is
by no means dependent on self-induced movement as implied in the theory. Stimulation of
receptors mediating the sense of position and
movement occurs with passive as well as active movement (Mountcastle & Powell, 1959).
Only the muscle spindles rely on self-induced
movement for their stimulation. Thus, following passive movement during the transformation phase, transfer would be expected to
occur to a passive posttransformation task.
Less transfer of learning would be likely to
occur from a task requiring passive movement to one demanding active movement, as
is common with tasks dissimilar in the type
of responses involved.
Felt-Position Hypothesis
Harris (1965) has proposed that changes
occurring with spatially transformed input
can be largely attributed to changes in the
position sense. He called the perception of
position "felt position" but made clear that
the hypothesis applies when position information is unconscious.
In discussing his own experiments, Harris
contended that since the observer was not
allowed to make any active movement with
his viewed arm during transformation, a simple motor-learning interpretation is inadequate to explain the changes which occurred.
But it is well established that even inspection
of a pictorial representation of a motor task
affects the learning of that task positively
(Gagne & Baker, 1950). In fact, Siipola
(1935) showed that subjects who observed
others performing the mirror-tracing task
benefitted from such perceptual pretraining.
In other words, it is not absolutely necessary
to perform the task to learn it; merely observing the arm or hand with spatially transformed vision is sufficient to result in some
learning. The argument advanced by Harris
that the elimination of active movement while
viewing the hand through a prism attests to
the inadequacy of a motor-learning explanation ignores the role of perceptual pretraining
in motor learning.
The second point, although more abstract,
is more fundamental in evaluating Harris's
SENSORY ADAPTATION AND BEHAVIORAL
explanation in terms of the felt position of
limbs or body parts. The study of perception
has long been as equally concerned with phenomena as it has been with responses. Descriptive phenomenology and behavioral psychophysics represent the extremes of a continuum of viewpoint with shades of approach
between. The study of learning, on the other
hand, has been almost exclusively concerned
with the objective observation, recording, and
measurement of responses. Subjective data
are seldom sought. But, and this is the core
of the argument, responses are accompanied
by sensory stimulation. The proprioceptive
end-organs function in the maintenance and
control of posture and movement. If a new set
of responses is learned, a new set of proprioceptive perceptual phenomena must also occur. If after many months of reaching to the
right to pick up the telephone, the position
of the telephone is altered to the left, the individual tends to continue reaching right. Gradually, he learns to reach left and the rightreaching response disappears. At the same
time there is a change in the proprioceptive
"sensation" accompanying the movements.
Reaching left is accompanied by different
felt-positions and felt-movements than reaching right. If with median plane transformation
the observer learns to reach right to pick up
an object which is actually straight ahead,
the learning which occurs has its sensory
component. To argue that changes in behavior with spatially transformed input are due
to changes in proprioception is merely to
offer an explanation in terms of the proprioceptive accompaniments of learning. If, as
Harris (1965) has indicated, this interpretation applies when position information is not
available to consciousness, then there would
seem to be little difference between the feltposition hypothesis and one couched in terms
of motor learning.
INTERACTION BETWEEN SENSORY AND MOTOR
PROCESSES: DIRECTION-CONTINGENT EFFECTS
The data treated so far strongly suggest
that under conditions of visual and auditory
transformation, independent sensory effects
and learning occur. While sensory spatial
adaptation is essentially similar to that occurring with appropriate nontransformed
COMPENSATION
319
stimulation, the learning effects are the same
in principle to those observable in a variety
of alternative situations involving perceptualmotor coordination. There is, however, a
third group of phenomena which, although
apparently related to the sensory effects, has a
motor component. These have been termed
"gaze-contingent" effects by Pick and Hay
(1966) and were first observed and intensively
studied by Kohler (1964).
The refraction of light passing through a
prism is a function of the angle at which light
enters the prism. If when viewing through a
prism the eyes are turned right, the transformation is different from that when the eyes
are turned left. The light falling on the eye
enters the prism at a different angle for each
direction of regard. Similar variations in the
characteristics of transformation occur when
the eyes remain in a fixed position and the
head is turned from right to left. Kohler
(1964) has shown that with prolonged wearing of prisms and other devices, adaptation
to these gaze-contingent transformations takes
place but that on removal, the consequent
aftereffect varies with direction of regard.
That is, if the eyes are turned one way, the
aftereffect is different from that when the
eyes are turned another. It is also claimed
that if spectacles whose eyepieces consist of
half blue and half yellow glass are worn for a
sufficiently long period, chromatic aftereffects
which are gaze-contingent also occur. This
latter observation was not confirmed by Harrington (1965), one of whose observers wore
the bicolor lenses for 146 days with no
marked changes.
Spatial aftereffects of the type studied by
Verhoeff (1902) and Gibson (1933) can be
induced either by transformation or by suitable nontransformed stimulation. The question
immediately arises concerning whether gazecontingent effects might also occur with nontransformed stimulation. Both Carlson (1964)
and Hein and Sekuler (1959) reported that
adaptation to curved and tilted lines can vary
as a function of eye position and head position with nontransformed vision. Carlson
(1964) proposed that perceptual adaptation
is basically an immediate localized consequence of stimulation but can become generalized through a process of conditioning by
R. H. DAY AND G. SINGER
320
temporal contiguity. Thus the data reported
by Kohler (1964) with transformed vision
and by Carlson (1964) and Hein and Sekuler
(19S9) with normal viewing indicate that
sensory spatial adaptation with aftereffect
under conditions of varying direction of gaze
may become associated with visual direction.
Another question concerns the possible occurrence of direction-contingent effects in
senses other than vision. Since sensory spatial
adaptation and aftereffect occur in the auditory modality, it is conceivable that directioncontingent auditory effects might also occur.
A pseudophone with a fixed sound source
would presumably result in variations in spatial and other properties with movements of
the head. In view of the possibility of such
effects occurring with sensory stimulation
other than vision, the term "direction-contingent" is preferable.
CONCLUSIONS
transformation of space they have been confused. Perceptual tests have frequently been
used to index motor learning, and behavioral
tests have been used to demonstrate perceptual changes. To further complicate the picture, Ivo Kohler and his colleagues have
shown and others (Pick & Hay, 1966) have
confirmed that varying visual transformation
with eye movement results in gaze-contingent
adaptation with aftereffect. Such directioncontingent effects may also occur with appropriate nontransformed stimulation. It is also
conceivable that such effects may take place
in hearing as well as vision.
Since behavioral compensation represents a
special case of motor learning, it is to be
expected that amount of practice, task difficulty, and other factors determine its cause
and dissipation. Experimental data have been
reported to show that this is so and that
the method of studying the learning process is
essentially a transfer-of-learning paradigm.
Over SO years ago a 180° rotation of the
frontal plane about its longitudinal axis was
used by Starch to demonstrate the course of
what he called "trial-and-error learning." This
technique has since been used in a large number of studies to demonstrate numerous features of the learning process, including that of
bilateral transfer. More recently, other classes,
directions, and extents of visual and auditory
transformation have been used to study similar changes in behavior. For the most part,
recent investigations have tended to regard
these behavioral changes as unique. An analysis of spatial transformation in terms of angular and parallel shifts in the median, horizontal, and slanted planes shows that there is
no difference in principle between frontal
plane transformations induced by mirrors and
median and horizontal plane changes induced
by prisms, lenses, and other optical systems.
In addition to motor learning under these
conditions, however, there also occur sensory
adaptive effects of a type essentially similar
to those which take place with appropriate
non transformed stimulation. With spatial
transformation, both effects may occur simultaneously and in the same direction. The basic
argument proposed here is that these effects
are independent but since they occur together
under conditions of optical and pseudophonic
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